1. Field of the Invention
The invention relates to a liquid crystal display device and more particularly, to a liquid crystal display device capable of having a uniform viewing angle while reducing the cost of production.
2. Description of the Related Art
Liquid crystal display (hereinafter, LCD) devices have trended to have increasingly wider application scope due to lightness, thinness, and low power consumption of these devices. Accordingly, LCD devices find use in office automation devices, audio/video devices and the like.
An LCD device adjusts the transmittance of a light beam in accordance with an image signal applied to multiple control switches arranged as a matrix in order to display desired pictures in a screen.
Since an LCD is not a self-luminescent device, the LCD requires a light source such as a back light unit. There are two types of back light units, i.e., a direct-below-type and an edge-type. The direct-below-type diffuses light generated from multiple lamps arranged in parallel at a rear surface of a diffusion plate by using the diffusion plate to irradiate the light. The edge-type irradiates light irradiated from a lamp through an incident surface provided at a side surface of a transparent light guide plate.
FIG. 1 shows a related art liquid crystal display adopting an edge-type back light unit that includes a main support 22, a backlight unit and a liquid crystal display panel 6 stacked inside of the main support 22. A top case 2 encloses the edge of the liquid crystal display panel 6 and the side surface of the main support 22.
The liquid crystal display panel 6 includes an upper substrate 3 and a lower substrate 5. Liquid crystal materials are injected between the upper substrate 3 and the lower substrate 5. The liquid crystal display panel 6 is provided with a spacer (not shown) to constantly keep a gap between the upper substrate 3 and the lower substrate 5. The upper substrate 3 of the liquid crystal display panel 6 is provided with a color filter, a common electrode and a black matrix (not shown). Signal lines such as a data line and a gate line (not shown) are formed on the lower substrate 5 of the liquid crystal display panel 6. A thin film transistor (TFT) is formed at an intersection between the data line and the gate line. The TFT switches a data signal to be transmitted from the data line to the liquid crystal cell in response to a scanning signal (i.e., a gate pulse) from the gate line. A pixel electrode is formed at a pixel area between the data line and the gate line. One side of the lower substrate 5 is provided with a pad area connected to each of the data line and the gate line. A tape carrier package (not shown), having a driver integrated circuit mounted thereon for applying a driving signal to the TFT, is attached onto the pad area. This tape carrier package applies a data signal from the driver integrated circuit to the data line. The tape carrier package also supplies the scanning signal to the gate line.
An upper polarizing sheet is attached onto the upper substrate 3 of the liquid crystal display panel 6, and a lower polarizing sheet is attached onto the rear side of the lower substrate 5 of the liquid crystal display panel 6.
The main support 22 is made of a molded material, and its inner wall is molded into a stepped coverage face. A securing part, in which the back light unit and the liquid crystal display panel 6 are disposed, is formed on the stepped coverage face. The back light unit and the liquid crystal display panel 6 are sequentially stacked on the stepped coverage face of the main support 22.
The back light unit, as shown in FIG. 2, includes a lamp 26 irradiating light onto the liquid crystal display panel 6, and a lamp housing 28 enclosing the lamp 26. A light guide plate 18 directs light incident from the lamp 26 toward the liquid crystal display panel 6. A reflection sheet 20 is arranged on a rear surface of the light guide plate 18. Optical sheets 10, 12, 14 and 16 are stacked over the light guide plate 18.
A cold cathode fluorescent lamp is mainly used as the lamp 26. The light generated from the lamp 26 is incident to the light guide plate 18 through a surface provided at one side of the light guide plate 18.
The lamp housing 28 includes a reflective surface in its inside, and the lamp housing 28 is installed to enclose the lamp 26, to thereby reflect the light generated from the lamp 26 toward the incident surface of the light guide plate 18.
The reflection sheet 20 is located at the rear surface of the light guide plate 18. The reflection sheet 20 serves to re-reflect light incident thereto, through the rear surface of the light guide plate 18, onto the light guide plate 18, thereby reducing the light loss. In other words, a portion of the light emitted from the lamp 26 progresses toward the rear surface of the light guide plate 18, and this light is then reflected from the reflection sheet 20 to progress toward the liquid crystal display panel 6.
The light guide plate 18 converts linear light provided by the lamp 26 into surface light, and the light guide plate 18 guides the light into the liquid crystal display panel 6. To this end, an inclined surface having a designated slant is formed on the rear surface of the light guide plate 18. Formed on the inclined surface are multiple printed patterns 52 spaced apart from each other by a designated distance. Accordingly, the light guide plate 18 makes light uniformly progress by being provided with a light receiving part toward an outgoing surface facing the liquid crystal display panel 6. Also, the light progressing toward the rear surface of the light guide plate 18 reflects from the reflection sheet 20, to thereby progress toward the outgoing surface.
The efficiency of the light incident to the liquid crystal panel 6 maximizes when the incident light is perpendicular to the liquid crystal panel 6. Accordingly, the optical sheets 10, 12, 14 and 16 convert the light outputted from the light guide plate 18 to be perpendicular to the liquid crystal panel 6, to thereby improve the efficiency of the light. To this end, a lower diffusing sheet 10 diffuses the light from the light guide plate 18 into an entire area, first and second prism sheets 12 and 14 change a progressing angle of the light diffused by the lower diffusing sheet to be perpendicular to the liquid crystal display panel 6, and an upper diffusing sheet 16 diffuses the light going through the first and second prism sheets 12 and 14. Accordingly, the light provided from the light guide plate 18 is incident, via the optical sheets 10, 12, 14 and 16, to the liquid crystal display panel 6.
Meanwhile, the related art liquid crystal display device supports the lamp housing 28 enclosing the lamp 26, and the device further includes a bottom cover 24 for radiating light generated from the lamp housing 28.
The bottom cover 24 includes a plane part and a side part perpendicularly bent with respect to each other in order to enclose a rear surface and a side surface at one side of the main support 22. A screw hole through which a screw (not shown) penetrates is formed at the side of the bottom cover 24. The bottom cover 24 is joined by the screw and is fixed in the main support 22.
The top case 2 is manufactured into a square band shape having a plane part and a side part perpendicularly bent with respect to each other. The top case 2 encloses the edge of the liquid crystal display panel 6 and the main support 22. The top case 2 is joined to the main support 22 by the screw (not shown).
As set forth above, the related art liquid crystal display device has the light incident from the lamp 26 to the light guide plate 18 progressing toward the liquid crystal display panel 6 through the use of the printed pattern 52. The related art liquid crystal display device tries to achieve a uniform distribution of light by using the printed pattern 52 formed on an inclined surface of the liquid guide plate 18. However, a disadvantage arises in that the viewing angle of light provided from the light guide plate 18, as shown in FIG. 3, cannot be uniform over the full angle. Further, the related art liquid crystal display device requires the upper and lower diffusing sheets 10 and 16, and the first and the second prism sheets 12 and 14 in order to adjust the direction of the light outputted through the light guide plate 18 and secure an uniform viewing angle at the full angular displacement. Accordingly, since the first and the second prism sheets 12 and 14 are necessarily employed in the related art liquid crystal display device, the production cost becomes high.
Further, since the light outputted from the light guide plate 18 irradiates via the first and the second prism sheets 12 and 14 to the liquid crystal display panel, a light loss occurs. As a result, the brightness lowers. Thus, the display quality of the liquid crystal display device is deteriorates from the lowered brightness.